Field of the Invention
This invention relates to an electroless Pb-Sn alloy plating bath composition.
Solder plating is used with electronic components, printed circuit boards and the like for corrosion prevention and component attachment.
The steady increase in the integration density of such electronic components and printed circuits has been accompanied by a tendency toward discrete conductor portions and finer circuit line features. As methods for conducting solder plating there are known the electrolytic solder plating method and the molten solder plating method. Both have drawbacks. Specifically, depending on the shape of the component etc. to be plated, the former method is not able to produce a uniform plating. Moreover, since it is not possible to secure electrical contact at portions that are not in electrical continuity, plating of these portions is impossible. On the other hand, the latter method is incapable of controlling, or ensuring the uniformity of, the plating thickness and is thus likely to result in the formation of electrical shorts, particularly in the case of fine patterns. It also entails heat-related problems such as heat-induced deformation of the substrate of the article being plated.
Prior Art Statement
As a result of research conducted for overcoming these shortcomings of the conventional methods, a number of electroless plating methods have been announced. The first plating bath compositions developed for these methods typically consisted of a mixture of stannous chloride, lead chloride, thiourea and hydrochloric acid, having a reducing agent etc. blended therewith. In a bath of such composition, reaction between the sparingly soluble metal chlorides and the thiourea produces addition compound precipitates. For ensuring that the precipitates dissolve completely so that the liquid remains transparent, the bath has to be maintained at a temperature of at least 70.degree. C. Even then, the aforesaid addition compounds tend to precipitate and separate from the liquid when the article to be plated is immersed therein and causes the temperature of the liquid surrounding it to decrease. Moreover, when electroless plating is conducted at the ordinary temperature using a bath of the foregoing temperature, the plating rate is on the order of only 1 micron per 10 minutes.
While the plating rate can be increased by raising the temperature of the bath composition, when this is done the surface of the plating film tends to become uneven and to be degraded by the deposition of coarse crystal grains. What is more, the plating film obtained reaches a thickness of no more than 3 microns.
For overcoming the defects of the aforementioned chloride bath, it has been proposed to use a borofluoride bath such as the bath comprising tin borofluoride, lead borofluoride, thiourea, borofluoric acid and a reducing agent disclosed in Japanese Patent Publication No. 62(1987)-2630 or, as disclosed in Japanese Patent Publication No. 62(1987)-17202, to use a bath comprising tin borofluoride, lead borofluoride, thiourea and a reducing agent whose concentration is varied between at least two levels during the electroless plating process. Being composed of borofluorides, the baths used in these methods are themselves toxic and pose a danger to those who work with them. They also require special treatment of the waste liquid and involve other problems regarding practical application.
In either the method using a chloride bath or that using borofluorides, the thickness of the film obtained when plating is conducted at 80.degree. C. for 15 minutes is on the order of only about 3 microns. The percentage of tin in the Pb-Sn alloy film obtained is too low to meet any of the S . A and B standards prescribed by JIS-Z3282. Nor does the film do well in fusing tests.
The inventors therefore conducted research for developing a plating bath composition that is free of the aforesaid defects of the prior art, specifically for developing a bath composition that does not require any special waste liquid treatment and which is capable of forming an electroless solder plating film on a copper surface with ease. As a result, they accomplished this invention.